Dining on the Fly

It's not every paparazzo who can catch a bug-eating bug in the act.

For a hungry insect, capturing and killing a delectable but uncooperative fellow bug is no mean feat. For a photographer, capturing the act on film is even harder, and recording all its gory, mesmerizing detail through an electron microscope is almost impossible: the problem is that the insects—both diner and meal—must be dead before they can be placed in the microscope, and dead or dying predatory insects tend to drop or vomit their prey.

A few years ago Stuttgart-based photographer Volker Steger found himself stung by the challenge. He was working as a photo editor for the German science magazine Bild der Wissenschaft, and he was asked to assemble pictures of insects used for biological control. Although electron micrographs of such bugs existed, there were none that showed them in the act of eating their prey—the pests that plague modern crops. So when the article was published in 1995, it was illustrated with ordinary, low-resolution photographs.

Afterward Steger set out to see if he could do better. The problem was easily stated: To prevent the bugs from loosening their grip on their victim—as an insect under attack will do—he had to kill them without their noticing that they were dying. The solution he devised led to the image of the predatory fly on the previous pages. Coenosia humilis is a tropical fly introduced into German greenhouses to feed on pests such as whiteflies and leaf miners, and it is already proving its worth as a biological control agent for use on flowers and crops.

Here it’s shown feeding on a fruit fly (Drosophila melanogaster). To photograph it in action, Steger starves a cageful of Coenosia humilis for a few days, then releases fruit flies into the cage. They catch the prey in midair, says Steger. And after the quarry is killed, Coenosia settles down to suck out the inner juices through its sharp proboscis. Steger waits until the predator has settled down on a pencil-like perch. Then he approaches it with an ether-soaked paintbrush. He says he must move slowly so as not to startle the fly. The ether makes it faint, so to speak, says Steger. Mean, eh? At first I believed they were dead, but they’re not. They take a long time to die, and I had a lot of cases of resurrection.

Once the predatory fly is well and truly dead, which usually takes half an hour or so, Steger can set about the serious business of drying and preparing it—the fly’s proboscis still perforating its victim—for viewing under the electron microscope. Using watchmaker’s tweezers, he lifts the predator by the tip of its wing while supporting its prey with a tiny brush, then dehydrates the pair in alcohol. The alcohol also coagulates the insects’ proteins, which keeps the fly’s proboscis firmly in place. To prevent the flies from collapsing, he immerses the two in liquid plastic, then dries them again. Finally he coats them with a thin layer of gold—one of the best conductors of electricity—and they’re ready for the electron microscope.

In the next photograph (left) we see the larva of a familiar insect: Coccinella septempunctata, commonly known as the seven-spotted ladybug. C. septempunctata is a hearty eater; both larvae and adults dine on all manner of aphids, from wheat-eating greenbugs to cotton aphids. These pests can do tremendous damage to crops grown in greenhouses, where they thrive in the warm, humid atmosphere. But the sap-sucking insects are no match for a ladybug larva, which is shown here draining the juices from the abdomen of Aphis gossypii, the cotton, or melon, aphid.

To photograph the feeding insect, Steger must ensure that the larva does not vomit all over its victim, as etherized ladybug larvae tend to do. Instead he freezes it. (This is a little torture chamber, isn’t it? he says.) First he houses the larvae in small petri dishes—keeping them in darkness because light disturbs them—and deprives them of food. Then he releases the hapless aphids into the dish. They attack them very fast, says Steger. Quickly, he puts a lid on the ladybug luncheonette and whisks it into the freezer—but not for too long, because he doesn’t want the insects to freeze completely. The ice crystals would destroy them—they would pop, he explains. At a few degrees above zero centigrade, though, they are almost dead, and they no longer react to the ether. Steger finishes them off and prepares them for viewing under the electron microscope by the same process used for Coenosia and its prey.

Below is the larva of Chrysoperla carnea, or green lacewing, which is using its large, hollow pincers to grab and suck the flesh from Aphis gossypii, leaving behind an empty shell. As adults, these predators are pale green with transparent green wings, shiny golden eyes, and long antennae; they feed on pollen, nectar, and the sugary honeydew exuded by aphids. The young, brown, alligatorlike larvae are far less delicate in their tastes; they consume moths, spider mites, leaf miners, and small caterpillars, as well as the aphids that infest eggplants, apples, sweet corn, and other U.S. crops.

Anaesthetizing the lacewing larva and its prey is easy, Steger says; he simply applies ether. The hard part is catching them at their feast. Since the larvae are tiny and active only at night, Steger spends hours in a darkened room, staring down a conventional microscope into a petri dish and waiting for the larvae to pounce on an aphid. Once they do, they suck it out so badly it looks like a collapsed Coke can, says Steger. Then they can be prepared for viewing.

At right is an adult ladybug swallowing an aphid whole. It’s already half-eaten—and if you’re very, very quiet, you can hear it munching, says Steger. They are really pigs. I don’t like killing animals, and I had an especially strong instinct against killing a ladybug—I could hardly do it. But this is the worst predator I have ever seen. It really is a killer. I think they can kill 10 to 20 aphids in a couple of hours.

Preparing the adult ladybug for electron microscopy presents the same problem as preparing the larva: confronted by ether, the ladybug vomits. Steger uses the freezer method again. He houses hungry adults in a petri dish, then supplies them with a smattering of aphid prey. To catch an aphid, the ladybug stands on four of its legs and uses its two front legs to grab it. Then it bites it and stuffs the aphid into its mouth with its forelegs, says Steger. Because C. septempunctata eats fast, Steger has to rush the covered petri dish into the freezer. The ladybug keeps munching for a little while, and then it gets slower and slower and slower until it stops, he says. The ladybug is then ready for ether and alcohol, liquid plastic and gold plating.

The results of Steger’s patience are not just pretty pictures. The electron microscope sometimes reveals secrets that can help biologists control pests. For example, the electron micrograph shows that Coenosia humilis does not insert its needlelike proboscis between the eyes of its prey, as was previously believed; rather, it siphons out the fruit fly’s flesh through a hole it has made with its proboscis on a soft part of the exoskeleton, between the head and the thorax. Information like that might help researchers decide whether an insect could serve as a biocontrol agent. A subtropical species of predatory fly with a weak proboscis might have been ignored in the past, but now that researchers know the fly penetrates the softest part of its prey’s exoskeleton, even weak-beaked bugs may be worth testing in biocontrol trials.

Still, although his photographs may add detail to biologists’ understanding of insect behavior, Steger’s real hope is that they’ll inspire ordinary gardeners. I would like to get many people interested in biological pest control, to create a consciousness of nature and of the balance between the predator and the prey in a healthy environment. You can buy those animals by mail order and use this approach in your living room, or in your yard, or in your greenhouse. I think that’s fantastic.